1. Field of the Invention
The present invention relates to an output circuit of a charge transfer device and, more particularly, to an improvement thereof for controlling the DC level of an output signal.
2. Description of the Related Arts
The charge transfer device, such as a charge-coupled device (CCD) transfers charges through a semiconductor substrate and along a channel on which a plurality of electrodes are aligned in a line through a thin insulator film. Charges transferred along the channel are detected in a form of voltage by introducing them into a floating diffusion region formed in contact with the channel. The floating diffusion region is formed in the semiconductor substrate as a diode region to operate as a capacitor element. The capacitor element produces a voltage in accordance with the quantity of the introduced charges. The produced voltage is amplified by a MOS source-follower circuit to form it into an electrical signal. The electrical signal is supplied to a clamping circuit to remove an effect thereon from the pulses for operating the CCD, and then used in various circuits using the output from the CCD. This output circuit has been demonstrated by M. H. White et al in a publication of "IEEE JOURNAL OF SOLID-STATE CIRCUITS" Vol. SC-9 (February, 1974), pp 1-12 and is shown in FIG. 1 of the attached drawings, in a simplified fashion.
The output circuit shown in FIG. 1 includes an output-detection circuit 20 and a clamp circuit 30. The output-detection circuit 20 is formed on a silicon substrate together with a charge transfer region 10 in which a channel is formed for transferring charges. Charges transferred through the channel are injected into a diode 27 which is formed as a floating diffusion region in the charge transfer region 10 in contact with the channel. A change in potential at an electrode P of the diode 27 is detected and amplified by a source follower circuit formed of transistors 22 and 23. Thereafter, the detected and amplified potential change is applied to the clamp circuit 30 having a capacitor 36, a transistor 35 and a buffer amplifier 4, thus obtaining an output, the DC level of which is clamped at V.sub.CL by the clamp circuit 30. It should be noted that a transistor 21 is a depletion type MOS transistor which functions as a reset transistor for the diode 27. The reference symbols RD and .phi..sub.R denote a reset drain terminal held at a constant potential and a reset pulse for operating the reset transistor 27, respectively. Further, AG denotes a bias voltage for controlling a DC current flowing through the source follower circuit.
The DC level of the output voltage V.sub.OUT from the output-detection circuit 20 is expressed as follows. If the threshold voltage of the transistor 22 is represented by V.sub.T2, the threshold voltage of the transistor 23 by V.sub.T3, the voltage at the terminal OD by V.sub.OD, the voltage at the terminal RD by V.sub.RD, the voltage at the terminal AG by V.sub.AG, the voltage gain .beta. of the transistor 22 by .beta..sub.2, and the voltage gain .beta. of the transistor 23 by .beta..sub.3, the following relationship holds upon condition that the transistors 22 and 23 in the source follower circuit operate in the saturation region: EQU I.sub.OD =.beta..sub.2 (V.sub.RD -V.sub.OUT -V.sub.T2).sup.2 =.beta..sub.3 (V.sub.AG -V.sub.T3).sup.2 ( 1)
and from the above expression, the following formula (2) is obtained: ##EQU1##
In the above-described output-detection circuit 20, however, the threshold voltages V.sub.T2 and V.sub.T3 of the MOS transistors 22 and 23 vary due to variations in the manufacturing process. Therefore, it has heretofore been impossible to adjust the level of the output DC potential V.sub.OUT as will be understood from the formula (2). Accordingly, it has heretofore been difficult to connect directly the charge transfer device with a peripheral circuit using the output V.sub.OUT. Therefore, it is necessary to interpose the clamp circuit 30 or the like as shown in FIG. 3 in order to control the DC level and to remove the effect from the pulses for operating the charge transfer region 10.
It is, therefore, a primary object of the present invention to provide an output circuit of a charge transfer device producing an output having a controlled amplitude irrespective of manufacturing errors.
It is another object of the present invention to provide an output circuit which is connectable to a circuit using the CCD output without interposing a clamp circuit.